Bicycles are preferably driven by means of a chain drive. Commonly the drive chain wheel connected to the crank arm lies predominantly outside the bicycle frame, while the output sprocket that is mounted on the rear-wheel axle is predominantly arranged inside the bicycle frame between the bicycle frame and the rear wheel.
Bicycles having smaller running wheels are preferably used so as to achieve small packing dimensions and weights, often designed as folding bicycles. Bicycles having 14 inch running wheels and less require a gear ratio of at least 1:5 between pedaling frequency and running-wheel frequency to be usable.
Trailer bikes are attached to the rear of a bicycle and, in contrast to those bicycles to which they are attached, mostly exhibit much smaller running wheels. Up till now, mostly transmissions having similar gear ratios are used for these running wheels as for the bicycles themselves. Thus the person riding the trailer bike has to ride at much higher pedaling frequencies than the person on the bicycle to which the trailer bike is attached. Even trailer bikes having small running wheels require the gear ratios mentioned above between the pedaling frequency and the running-wheel frequency.
So that the gear ratios that have been mentioned can be achieved with a single-stage chain drive, an output sprocket having as few teeth as possible has to be used. All tooth counts that have been mentioned below in this document, of the output sprockets and drive chain wheels refer to the chain pitch of 12.7 mm that is conventional for bicycle chains.
In the following solutions, output sprockets having low tooth counts are used:
a. In most cases the output sprocket is arranged between the bicycle frame and the rear wheel, the output sprocket being supported on a rear-wheel axle that is firmly connected to the bicycle frame. This solution has the disadvantage that on account of the strength requirements on the rear-wheel axle resulting from the bending loads, this axle must exhibit across its entire length a minimum diameter, on which the output sprocket is mounted, as a result of which the tooth count of the output sprocket amounts to at least nine teeth. This solution also has the disadvantage that the required freewheel has to be arranged in the rear-wheel hub or on the bottom bracket. JP 2003-054 479 A discloses a chain drive with an output sprocket having only eight teeth. Here it is a disadvantage that, on account of the system, no freewheel clutch is provided on the rear wheel. It would then have to be disposed between the crank arm and the drive chain wheel, seen in the power flow.
b. JP 11-321 768 A discloses a chain drive that uses an output sprocket that is arranged outside the bicycle frame and exhibits only seven teeth. A disadvantage of this solution is that the output sprocket exhibits an outer thread that is screwed directly to the rear-wheel shaft and that a freewheel clutch is arranged between the bottom bracket and the drive chain wheel, as a result of which no commercially available drive chain wheel directly connected to a crank arm can be used and as a result of which the output sprocket, the drive chain wheel, and the chain co-rotate in the freewheel case.
c. DE 101 27 614 A1 discloses a chain drive having an output sprocket of fewer than eleven teeth respectively having seven teeth. It is a disadvantage that the rear wheel is rotationally engaged with the chain and no freewheel clutch is provided.
d. JP 2005-289 283 A discloses a one-armed rear-wheel suspension on the bicycle frame, the output sprockets of the chain drive being mounted on both sides in the bicycle frame that has been designed as the gear case, and the rear wheel being arranged outside the gear case on one side of the bicycle frame. A freewheel clutch is arranged between the output sprockets and the rear-wheel shaft. So that a chain line distance that is conventional for bicycles is achieved for the bicycle drive, seen axially, the bicycle frame is designed between the output sprockets and the rear wheel as a thin-walled gear case. The consequence is that, seen axially, the rear-wheel shaft and the bottom-bracket shaft have to be mounted on both sides of the output sprocket respectively of the drive chain wheel relative to the gear case, in order to achieve the required stiffness between the rear-wheel shaft and the gear case respectively between the bottom-bracket shaft and the gear case. It is a disadvantage that in the process the rear-wheel shaft has to absorb, in the area of the freewheel clutch, the bending moments that are introduced from the rear wheel and are caused by the weight, and has to exhibit a minimum diameter that complies with the strength requirements and that in the process the smallest output sprocket of the sprocket assembly exhibits at least 13 teeth. It is further a disadvantage that on account of the fact that the rear-wheel shaft and the drive chain wheel are mounted in a supporting gear case on both sides, no commercially available drive chain wheel that is directly connected to a crank arm, can be used.
US 2006/0 108 858 A1, US 2006/0 108 859 A1 and BE 558 090 A reveal rear-wheel drives where a bearing supports a rear-wheel shaft relative to the frame and, seen in the power flow, a freewheel is arranged between a driving mechanism and the rear-wheel shaft, the freewheel being installed as a complete unit on the rear-wheel shaft. The freewheel comprises a freewheel clutch and bearing that supports the driving mechanism on the rear-wheel shaft.
A disadvantage of these embodiments is the fact that the bearings of the freewheel support the driving mechanism in each case relative to the rear-wheel shaft and, seen radially, are thus arranged between the driving mechanism and the rear-wheel shaft. On account of the design, the freewheel mechanism arranged between these bearings can thus again, seen radially, be implemented only between the sprockets of the driving mechanism and the rear-wheel shaft. In addition, the complete freewheel unit is connected to the rear-wheel shaft using a shaft-hub-connection. The radial installation space required for these elements in the area of the bearings, of the freewheel clutch and of the shaft-hub-connection probably does not permit an implementation of a driving mechanism having an output sprocket of fewer than 11 teeth in this area on account of the strength that is required for bicycles. It is in particular the bearing, which supports the driving mechanism, that is responsible for these disadvantages that, seen axially, is arranged between the freewheel mechanism and the bearing that supports the rear-wheel shaft relative to the frame. This bearing prevents an output sprocket having a lower tooth count and an acceptable chain line distance being implemented on the driving mechanism.